Effects of Resonator Volume on The Oscillator Near-Carrier Phase Noise

This paper reports the first study of near-carrier phase noise (PN) dependency on the resonator volume/harmonic-number in thin-film piezoelectric-on-substrate (TPoS) MEMS oscillators. Two lateral bulk-acoustic TPoS resonators with different volumes (3rd and 5th harmonic orders) were designed to operate at the same ∼25.6 MHz and fabricated on a $25\mu\mathrm{m}$ silicon on insulator (SOI) substrate. The devices chosen for this study have different volumes but exhibit identical Quality factor ($Q$) and insertion loss (IL). The measured PN of the oscillators built based on these two resonators are measured and it is observed that the one based on the 5th order resonator exhibits ∼6dB lower PN @1KHz frequency offset compared to the 3rd order device. Although our preliminary results are not conclusive enough, it is hypothesized that an inverse proportionality to the resonator volume/mass predicted by earlier models could also describe this observation.